1. Field of the Invention
The present invention relates to a light-emitting device using an organic compound, and more specifically, to an organic light-emitting device that emits light by applying an electric field to a thin film composed of an organic compound.
2. Description of the Related Art
An organic light-emitting device suggests its potential to find use in a wide variety of applications owing to its characteristics such that the device can be formed into a thin, light-weight light-emitting device which shows high luminance at a low applied voltage; has a variety of emission wavelengths; and provides high-speed response. However, particularly when considering its application to, for example, a full-color display, the stability of the existing device is not sufficient for putting into practical use, so that an additional improvement in stability has been needed.
Japanese Patent Application Laid-Open No. 2002-359080 discloses a method of improving the durability in continuous drive of a device by incorporating therein a light-emitting material and a triplet capturing agent to reduce the triplet excited state of the light-emitting material.
Further, Applied Physics Letters, Volume 75, Number 6, (1999) discloses doping a hole-transporting layer with rubrene to improve the durability in continuous drive of a device. However, according to this technique, the rubrene contained in the hole-transporting layer emits a strong yellow light, so that the device cannot be applied to a light-emitting device for emitting a green light or blue light.
One of the causes for the degradation of an organic light-emitting device by driving is that a hole-transporting material which constitutes a hole-transporting layer is degraded by going through a triplet excited state. That is, when electron leakage from a light-emitting layer to the hole-transporting layer occurs, holes and electrons recombine in the hole-transporting layer, so that excited singlet states and triplet excited states are generated at a probability ratio of 1:3. Therefore, the triplet excited state generated in a larger amount is important in the context of the degradation. Moreover, the triplet excited state generally has a long lifetime of an order of ms or more, so that degradation such as decomposition due to going through a triplet excited state and the like becomes a problem.
In addition, in order that excitons are efficiently formed in a light-emitting layer to thereby allow the layer to emit light, the energy gap E1 of a main component which constitutes the light-emitting layer and the energy gap E2 of a main component which constitutes a hole-transporting layer satisfy at least the relationship of E1<E2. However, in cases where a material having a large energy gap E1 is used as is the case where the emission color is blue, such energy relationship is sometimes not satisfied, so that the degradation of the hole-transporting material becomes significant.